1. Field of the Invention
The present invention relates to a containment system for containing a burst impeller or impeller fragments and, in particular, an impeller containment system that utilizes a cooperating snagger and catcher to contain the tangential movement of a burst impeller or impeller fragments.
2. Description of the Prior Art
Aircraft engines and auxiliary power units are known to include turbines and compressors which rotate about a central shaft at high rotational speeds. Although modern gas turbine engines and auxiliary power units can perform for thousands of hours without any significant malfunction, circumstances have arisen where turbine blades or impeller blades can burst from their rotational hub. In general, when such a burst occurs, it is most desirable to contain such hub fragments of the burst disk within the shortest possible radius. In this way, the material cross-section required to provide the necessary shear and hoop strength to contain the burst can be minimized to reduce weight, while still maintaining a sufficient factor of safety for protection of the engine and aircraft systems and structure.
It is also desirable to contain the burst within the shortest axial distance as can be obtained, so that the range of trajectories of burst fragments resulting from a burst will be minimal. In practice within the prior art, impeller hub fragments have been contained by using impeller shrouds, diffusers, and the outer engine cases as the primary containment structures. However, the conventional prior art impeller shrouds and diffusers have sometimes failed to contain the hub burst over sufficiently small axial and radial distances. In the conventional prior art, these containment structures have tended to be remote from the respective origin of burst. Consequently, in order to span the anticipated range of trajectories of burst fragments, these containment structures have been axially long and radially thick such that their cross-sections have been massive relative to adjacent normal engine structure. Tests have shown that these structures can be inefficient with respect to their weight.
An undesirable feature of these structures is that the burst fragments are generally unrestrained until they have reached the outer structure of the engine, by which time the fragments can be unfavourably oriented for efficient containment having been deflected from the original plane of rotation, both by intermediate structure and by the mode of break-up of the disk since each failure event can be different.